Optical transmission systems based on optical fibers are being used to transmit relatively large amounts of information in numerous applications such as telecommunications and data networking. With a growing demand for faster broadband and more reliable networks, optical communication systems based on optical fibers face important challenges.
Optical fibers represent optical waveguides that guide electromagnetic waves in the optical spectrum. The propagation of the waves along an optical fiber depends on several parameters related to the fiber such as its geometry, its mode structure, the distribution of the refractive index, the material it is made of, etc. Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction. The cladding is such that the light launched into the core remains in the core. The optical fiber, which acts as a waveguide, guides the light launched into the fiber core. When the light launched into the core strikes the cladding, it undergoes a number of internal reflections.
There exist two types of optical fibers: multimode optical fibers and single-mode optical fibers. The difference between these two types of fibers lies on the number of modes allowed to propagate in the core of a fiber. As used herein, a “mode” refers to an allowable path (light propagation path) for the light to travel down a fiber.
A multimode fiber allows several modes while a single-mode fiber allows only one mode.
In a multimode fiber, the time taken by light to travel through a fiber is different for each mode, which results in a spreading of the pulse at the output of the fiber (which is referred to as “intermodal dispersion”).
The difference in the time delay between the modes is called Differential Mode Delay (DMD). Intermodal dispersion limits multimode fiber bandwidth. A fiber's bandwidth determines the information carrying capacity of the fiber, which includes how far a transmission system can operate at a specified bit error rate and the upper bound on the rate at which information can be reliably transmitted over the optical transmission channel. By limiting the fiber bandwidth, intermodal dispersion reduces the information rate that can be achieved with arbitrarily small error probability.
A single-mode fiber does not present intermodal dispersion and has higher bandwidth than multimode fiber. A single-mode fiber allows for higher data rates over much longer distances than achievable with a multimode fiber.
Although single-mode fiber has higher bandwidth, multimode fiber supports high data rates at short distances. Multimode fibers are consequently particularly used in shorter distance and in cost sensitive LAN applications. Multi-mode fibers allow the propagation of many modes in a single-core or in multi-core fibers where each core can be single-mode or multi-mode. The various propagation modes form a set of orthogonal channels over which independent data symbols can be multiplexed. Space Division Multiplexing (SDM) techniques such as Mode Division Multiplexing (MDM) can be used to perform such multiplexing, which results in an increase of the link capacity by a factor corresponding to the number of propagation modes. Since Wavelength division Multiplexing (WDM) systems are approaching the nonlinear Shannon limit, Space division multiplexing (SDM) holds the promise to increase the capacity of the optical transmission links.
Multimode fibers can offer higher transmission rates than their single-mode counterparts. However, taking advantage of the presence of multiple modes to multiplex and transmit larger amount of data symbols requires managing several modal detrimental impairments. These impairments are mainly due to imperfections of the optical components (e.g. fibers, amplifiers and multiplexers) and to the crosstalk effects between the various propagation modes. Such imperfections induce non-unitary impairments, i.e. impairments that cause a loss of orthogonality and/or a loss of energy between the different channels over which independent data symbols are multiplexed. Such impairments can significantly reduce the capacity of the optical links and deteriorate the performance of the transmission system, particularly in long distances applications.
The bandwidth of a multimode fiber is generally higher than that of single-mode fibers, each mode being separately modulated and the signal to be transmitted being multiplexed on different modes. This bandwidth is limited by the coupling between modes during propagation (“crosstalk inter-mode”).
In addition, for long distances, amplifiers are needed between the optical fiber sections. As a result of the modal dispersion of the amplifier gain, the modes do not undergo the same attenuation. Other components, such as optical multiplexers or demultiplexers for example, as well as imperfections in the fiber may further impact the attenuation. The differential loss between modes, also called MDL (acronym for “Mode Division Multiplexing”), induces increased sensitivity to noise sources, thereby limiting the scope of these systems.
Multicore fibers comprise a plurality of cores (usually 2 to 7 cores) within a common cladding. The small size of the cores only allows single mode propagation in each of them. Unlike multimode fibers, multicore fibers do not present modal dispersion. In contrast, the evanescent waves create a coupling between the different cores (inter-core crosstalk), the level of crosstalk being all the higher as the number of cores is high and the inter-core distance is low. Such crosstalk affecting propagating modes through multi-mode fibers is also known as Mode Dependent Loss (MDL). MDL effects require either optical or digital signal processing solutions to be reduced.
The impact of MDL effect is detrimental to channel capacity as disclosed in a number of studies, such as:    P. Winzer et al., “MIMO capacities and outage probabilities in spatially multiplexed optical transport systems,” Opt. Express, 2011;    C. Antonelli et al., “modeling and performance metrics of MIMO-SDM systems with different amplification schemes in the presence of mode-dependent loss,” Opt. Express 23, 2203-2219 2015.    A. Lobato et al., “On the mode-dependent loss compensation for mode-division multiplexed systems,” (ICTON), 23-27 Jun. 2013.
A proposed solution to mitigate the MDL effect has been described in K. Ho et al, “Mode-dependent loss and gain: statistics and effect on mode-division multiplexing,” Opt. Express 19, 16612-16635 (2011). According to this approach, strong mode coupling is used to reduce MDL and modal dispersion.
In other approaches, it is known to use mode scrambling to couple modes at local points. Existing mode scramblers are designed to solve problems of bandwidth measurement reproducibility in multi-mode fibers. In particular, some mode scramblers (also called ‘mode mixers’ or ‘mode couplers’) were proposed to provide a uniform input mode power distribution.
A mechanically-induced implementation of mode scrambler may be used, as disclosed in:    “M. Tokuda, S. Seikai, K. Yoshida, N. Uchida, Measurement of Baseband Frequency Response of Multimode Fibre By Using a New Type of Mode Scrambler, Electronics Letters, Volume: 13, Issue: 5, March 1977”, and    “A. Li, A. Al Amin, X. Chen, and W. Shieh, Transmission of 107-Gb/s mode and polarization multiplexed CO-OFDM signal over a two-mode fiber, Optics Express Volume: 19, Issue: 9, pages 8808-8814, 2011”;
Mechanically-induced scrambling designs are based on a mechanical perturbation in the form of the fiber via different ways such as sinusoidal serpentine bends, gratings, and joining fibers having different profiles.
Another way to implement mode scrambling is based on the use of optical components such as mode converters.
MDL mitigation based on mechanically-induced mode scrambling was addressed in some conventional implementations. For example, in S. Warm et al., “Splice loss requirements in multi-mode fiber mode-division-multiplexed transmission links,” Opt. Express 21(19), 2013, a random mode permutation is added after a certain number of fiber splices in order to reduce the correlation of modal coupling. In FR3023436, an optical fiber transmission system equipped with random scramblers has been proposed to switch propagation modes or cores. Such a system comprises a space-time encoder and a plurality of modulators respectively associated with separate propagation modes or cores of the fiber, each modulator modulating a laser beam. The fiber comprises a plurality of sections, an amplifier provided between any two consecutive sections of the optical fiber, and a mode switch associated with each amplifier in order to switch the modes between at least two consecutive sections. A mode scrambler is associated to each amplifier for randomly permuting the modes between at least two consecutive sections. However, the random permutation of the modes is not sufficient to average the MDL effect experienced by all the propagation modes. More generally, existing approaches based on the use of scramblers require an important number of mode-scramblers (one scrambler for each amplifier) and provide limited performance.
There is accordingly need for an improved scrambler.